As light sources of liquid crystal displays, backlight devices of the edge light type or direct type are conventionally used. Since backlight devices of the edge light type themselves can be manufactured with a small thickness, they are used for notebook computers etc., whereas backlight devices of the direct type are frequently used for large-sized liquid crystal televisions etc.
Lights projected from these backlight devices contain components projected along directions inclined from the front direction. Lights projected from backlight devices of the edge light type, in particular, contain a lot of components projected along directions significantly inclined from the front direction, and thus it is difficult to obtain high front luminance.
Therefore, in order to direct lights projected at an angle with respect to the front direction to the front direction to improve front luminance, a light control film for condensing lights from a backlight device is incorporated.
For example, a prism sheet is conventionally used as one of the light control films that can achieve such an aspect. The prism sheet has a fine regular structure having a size of about several tens of micrometers, and although it shows high light condensing property, it generates glare, and suffers from a problem that, when it is incorporated into a backlight device, a moiré pattern occurs between the prism sheet and pixels of liquid crystal panel to degrade image quality. Therefore, by placing a light-diffusing film on the prism sheet, generations of glare of light source and moiré pattern are suppressed. However, if a light-diffusing film is placed, front luminance is reduced, and there also arises a problem that the number of parts increases.
As a light control film different from the prism sheet, there is a light control film having uneven profile formed on the surface thereof, which can improve luminance of a backlight device for the front direction, and enable uniform projection of lights from the backlight device.
In order to direct lights to the front direction as much as possible, the light control film of this type is preferably provided with an uneven pattern consisting of convexes arranged on the surface of the light control film without gaps, which convexes are suitable for improving property of condensing lights to the front direction of the backlight device. As such convexes for improving light-condensing property, for example, fine convexes having approximately circular bases are proposed in Patent document 1. Such convexes are formed by, for example, photolithography method, printing method, or the like (Patent documents 1 and 2).
When fine convexes having approximately circular bases are formed by photolithography, the convexes are preferably those having bases of approximately the same diameters for forming specific shapes showing high light-condensing property by one process. The best way for enhancing front luminance with such convexes is the closest packing of the convexes wherein the convexes are arranged without overlaps and gaps. However, such an arrangement constitutes a regular structure as in the example of the prism sheet mentioned above.
If a light control film having such a regular structure is incorporated into a backlight device, there arises a problem that a moiré pattern occurs between the film and a member having a regular structure such as pixels of a liquid crystal panel to degrade image quality, although it improves front luminance.
In order to prevent such generation of a moirépattern, an uneven pattern in which convexes having bases of substantially the same diameters are arranged without regularity is required. However, for example, if convexes are randomly arranged by using random numbers or the like so that the convexes should not overlap with one another, the ratio of the bases of the convexes (packing ratio) is about fifty-odd percents at most. In this case, although the problem of the moiré pattern can be actually solved, the packing ratio of the convexes of the uneven pattern is extremely lower than the packing ratio of the closest packing (hexagonal arrangement), 90.7%. Therefore, the light-condensing property for the front direction is markedly reduced, and if such a pattern is used for a member of a backlight device, the front luminance of the backlight device becomes low.
In order to prevent generation of a moiré pattern, such a design is also possible that an uneven pattern constituted by convexes regularly arranged as the closest packing is used with a light-diffusing film separately incorporated into a backlight device. Although the generation of a moiré pattern can actually be prevented by this method, lights directed to the front direction are diffused with the light-diffusing film, and thus front luminance is reduced. Moreover, it increases the number of parts constituting the backlight device at the same time, and thus it is not an effective means.    Patent document 1: Japanese Patent Unexamined Publication (KOKAI) No. 2004-33811 (Modes for Carrying out the Invention)    Patent document 2: Japanese Patent Unexamined Publication No. 2003-270412 (Related Art)